This invention relates to an intake manifold for internal combustion engines, and more particularly to improvements in the structure of the auxiliary intake passage system of an internal combustion engine.
An intake manifold for an internal combustion engine is well known which is arranged between a carburetor unit and the main body of the engine and is formed therein with a plurality of main intake passages for feeding a lean mixture to respective main combustion chambers of the engine, a main distributing chamber from which diverge the main intake passages, a plurality of auxiliary intake passages for feeding a rich mixture to respective auxiliary combustion chambers of the engine, and an auxiliary distributing chamber from which diverge the auxiliary intake passages, the above main distributing chamber being divided by a partition wall located at the mixture-inlet portion of the same chamber into a primary distributing chamber for distributing a mixture to the main intake passages for low-load operation of the engine and a secondary distributing chamber for distributing a mixture to the main intake passages for high-load operation of the engine.
In such type intake manifold, as known from Japanese Provisional Patent Publication (Kokai) No. 53-57315, mixture-inlet ports leading, respectively, to the primary distributing chamber, the secondary distributing chamber and the auxiliary distributing chamber are usually arranged in a line extending substantially normally to the plane of juncture between the intake manifold and the main body of the engine so as to obtain even distribution of the mixtures to the main combustion chambers and the auxiliary combustion chambers. Due to the limited size of the intake manifold, the main distributing chamber and the auxiliary distributing chamber are disposed to overlap with each other in a vertical direction of the intake manifold with the latter located inwardly of the former. This arrangement necessitates locating the mixture-inlet port leading to the auxiliary distributing chamber through the surface of a thick lateral wall of the main distributing chamber such that the mixture-inlet port communicates with the auxiliary distributing chamber by way of an intake guide passage which is formed through the above lateral wall of the main distributing chamber. That is, the intake guide passage extends around the main distributing chamber, and therefore its internal flow path is inevitably long and complicate in configuration, providing the possibility of uneven distribution of the mixture from the auxiliary distributing chamber to the auxiliary intake passages due to increased flow resistance and turbulence in the intake guide passage, etc.
Further, the aforesaid linear arrangement of the mixture-inlet ports leading to the primary and secondary distributing chambers and the auxiliary distributing chamber necessitates prolonging the size of the carburetor-mounting surface on the outer surface of the intake manifold in a direction in which are arranged the mixture-inlet ports. This in turn necessitates designing the intake manifold and a carburetor unit mounted on the mounting site large in size.